Crisot has made available a new demonstration video (xvid avi mirror1|mirror2 MPEG1 mirror1|mirror2) showing the AmigaOS4-beta implementation of the unique AmigaOS feature, screen dragging. For recent user screenshots of the AmigaOS4 pre-release look here.

But there's no special operating system support for them, right? The PPC or x86 just acts as a kind of coprocessor?

That's correct. PPC and 68k apps run on this hardware platform perform poorly, because you need to add-on memory management (WarpUP or PowerUP) between those cpus. The performance is not optimal, because of task switching penalties when switching between AmigaOS and WarpUP/PowerUP operation.

AmigaOS4 turned this around by adopting the PPC as main CPU and emulating the 68k, handling all this inside the kernel, making performance much faster.

Thus the 68k is not functional on those PPC A1200/A4000's that run the AmigaOS4 prerelease.